Systems for the delivery of pharmaceutically beneficial agents are well known in the art. Dispensing systems which deliver their contents by diffusion through a permeable polymer coating or wall are well known but suffer from severe limitations. For example, many pharmaceutically beneficial agents cannot be delivered from such diffusion controlled devices. In many instances permeation rates through the permeable polymer coating are inadequate to provide useful concentrations of the beneficial agent. In many others, the beneficial agent has such a high molecular weight that it will not diffuse through the polymer coating.
Also well known are delivery systems which operate by means of an osmotic pumping mechanism. In a typical delivery system of this type the beneficial agent is contained within a continuous, semipermeable film, e.g. a capsule or a film coating, having a hole of predetermined size drilled therethrough. In operation the delivery system is placed in the appropriate aqueous environment, e.g. the stomach or cul-de-sac of the eye, whereupon it .[.imbides.]. .Iadd.imbibes .Iaddend.water through the semi-permeable film, thereby dissolving at least in part the contents of the delivery system. This causes an increase in the internal (osmotic) pressure, which results in dissolved contents being continuously pumped out of the delivery system through the hole at a controlled rate over a predetermined period of time.
The devices taught in the art have a number of deficiencies. They are, in most case, complex devices having multiple parts or requiring special fabrication steps. For example, many require the drilling of a hole through the film coating of each device. Consequently, the delivery system itself is relatively expensive to fabricate and contributes substantially to the final cost of the product.
Prior art devices each having a single passageway through which their dissolved contents are delivered have other advantages. When the beneficial agent contained and delivered by such a device is irritating to the biological tissue in the region in which the device is used, local tissue irritation could be a problem at the locus of delivery of the agent, i.e. in the vicinity of the hole through which the concentrated agent is pumped.
Alternatives to drilling individual holes through the semipermeable film have been disclosed. For example, the use of a friable, inexpandable wall has been described which fractures when it .[.imbides.]. .Iadd.imbibes .Iaddend.water to provide cracks and fissures through which the contents are then delivered. Passageways provided by erosion of bioerodible fibers incorporated in the wall of the delivery device have also been described.
There is a very large body of art describing the various delivery systems. The U.S. patents below are cited as representative of the controlled delivery system art: U.S. Pat. Nos. 3,845,770; 3,916,899; 4,016,880; 4,160,452 and 4,200,098.